Within some therapy areas the tendency of a patient to adhere to the prescribed therapy is dependent on the simplicity of the specific treatment regimen. For example, many people with type 2 diabetes are diagnosed with the disease at a relatively high age where they are less prone to accept a treatment that intervenes too much with their normal way of living. Most of these people do not like constantly being reminded of their disease and, as a consequence, they do not want to be entangled in complex treatment patterns or waste time on learning to operate cumbersome delivery systems.
Basically, people with diabetes need to minimise their glucose excursions. Insulin is a well-known glucose lowering agent which must be administered parenterally to be effective in the body. The presently most common way of administering insulin is by subcutaneous injections. Such injections have traditionally been performed using a vial and a syringe, but in recent years so-called injection devices, or injection pens, have gained more and more attention in the marketplace. Many people find these injection devices easier to handle and generally more convenient than the vial and syringe solution. For example, because an injection pen carries a prefilled drug container the user is not required to carry out a separate filling procedure before each injection.
In some prior art injection devices which are suitable for self-injection, the user has to set a desired dose using a dose setting mechanism of the injection device and subsequently inject the previously set dose using an injection mechanism of the injection device. In this case the dose is variable, i.e. the user must set a dose which is suitable in the specific situation each time a dose is to be injected.
Other prior art injection devices are adapted to inject a fixed dose each time it is operated. In this case the user has to prepare the injection device, thereby setting the fixed dose, using a dose setting or dose arming mechanism, and subsequently inject the dose using an injection mechanism.
Regardless of the specific type of injection device it is usually recommended that the user primes the device before an injection to ensure that the amount of drug delivered during the injection actually corresponds to the dose that has been set. For variable dose injection devices priming can be performed by setting a small dose and expelling it outside the body. A piston rod is thereby advanced a small distance in the injection device to cause a piston to pressurise the contents of the container. A small volume of drug is wasted as a result of this action, but the user is then guaranteed both that the piston rod is in operative contact with the piston and that the delivery path has been emptied of air. For fixed dose injection devices, however, such a priming action normally results in wastage of a much larger volume of the drug since the injection device will deliver a specific, therapeutically relevant, dose each time the injection mechanism is activated.
EP 0 937 477 B1 discloses a medication delivery pen which includes a priming control mechanism allowing the user to perform a manual priming by rotating a sleeve about a central part of the pen.
It is desirable to provide an injection device which is simple to handle and which is intuitive and easy for the patient to learn how to use. In particular, it is desirable to provide an injection device which is capable of accurately administering one or more doses of liquid drug, while at the same time requiring a minimum number of operations to be performed by the user. In that respect, it is desirable to provide an injection device which does not require the user to perform a manual priming operation before an injection. Furthermore, specifically in relation to fixed dose injection devices, or other types of injection devices which does not offer user adjustable dose setting, such as e.g. some single shot injectors, it is desirable to avoid wasting a relatively large volume of drug when priming the injection device.